[LLVMbugs] [Bug 1385] NEW: Miscompilation of t-pow in GMP testsuite

[bugzilla-daemon at cs.uiuc.edu]

http://llvm.org/bugs/show_bug.cgi?id=1385

           Summary: Miscompilation of t-pow in GMP testsuite
           Product: libraries
           Version: trunk
          Platform: Macintosh
        OS/Version: MacOS X
            Status: NEW
          Severity: normal
          Priority: P2
         Component: Backend: PowerPC
        AssignedTo: unassignedbugs at nondot.org
        ReportedBy: isanbard at gmail.com


There seems to be a problem with how code is generated for a call after a loop is unrolled. Consider 
this C code (on a PowerBook G4 machine):

typedef unsigned long int mp_limb_t;

typedef struct {
  int _mp_alloc;
  int _mp_size;
  mp_limb_t *_mp_d;
} __mpz_struct;

typedef __mpz_struct mpz_t[1];
typedef const __mpz_struct *mpz_srcptr;
typedef __mpz_struct *mpz_ptr;

void Foo(mpz_srcptr base) {
  unsigned i;
  mpz_t want;
  __gmpz_init(want);
  for (i = 0; i < 2; i++) {
    __gmpz_mul(want, want, base);
  }
  __gmpz_clear(want);
}

void Bar(mpz_srcptr base) {
  mpz_t want;
  __gmpz_init(want);
  __gmpz_mul(want, want, base);
  __gmpz_mul(want, want, base);
  __gmpz_clear(want);
}

If you look at "testcase.llvm.s", you'll notice that, even though both Foo and Bar functions compute the 
exact same thing, the Foo function, because it's a loop that's been unrolled, has an "implicit def" of R3 
before the gmpz_clear call:

        mr r3, r29
        mr r4, r29
        mr r5, r30
        bl L___gmpz_mul$stub
        ;IMPLICIT_DEF_GPRC r3                                                                             
        bl L___gmpz_clear$stub

while the Bar case has:

        mr r3, r29
        mr r4, r29
        mr r5, r30
        bl L___gmpz_mul$stub
        mr r3, r29
        bl L___gmpz_clear$stub

The Foo case would be fine, if R3 wasn't trashed in the calls, but it appears to be. Though my guess is 
that it probably should be marked as "clobbered" across calls. The "mpz_clear" function's in a library 
and looks like this:

void mpz_clear (mpz_ptr m) {
  (*__gmp_free_func) (m->_mp_d, m->_mp_alloc * BYTES_PER_MP_LIMB);
}

Now, gcc produces code for Foo that looks like this:

L2:
        addi r3,r1,56
        mr r5,r29
        mr r4,r3
        bl L___gmpz_mul$stub
        addic. r30,r30,-1
        bne- cr0,L2
        addi r3,r1,56
        bl L___gmpz_clear$stub

and Bar that looks like this:

        addi r3,r1,56
        mr r5,r29
        mr r4,r3
        bl L___gmpz_mul$stub
        addi r3,r1,56
        bl L___gmpz_clear$stub

So it's doing the correct thing. We should too.

BTW, this is the only test which failed in the GMP testsuite. Woo!

-bw



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